1. Field of the Invention
The present invention relates to a high-frequency switch, and more particularly to a high-frequency switch that is used to provide RF signal switching for radars and communication apparatuses operating in a millimeter wave band.
2. Description of the Related Art
The application of a microwave device increasingly progresses in accordance with an increase in the degree of computerization. Microwave devices are applied to various fields. For example, there are microwave devices for use in cellular phones, which use a relatively low frequency, microwave devices for use in high-frequency telecommunication, and microwave devices for use in telecommunication equipment such as high-frequency vehicle-mounted radars and observation satellites.
Recently, a small-size, high-speed, high-frequency switch based on a microwave IC (MIC) was developed as a high-speed switch. Further, an MMIC (Monolithic Microwave IC), which is obtained when a connection path and switching element are rendered integral with each other and formed on a semiconductor substrate, is employed so as to raise the speed of the switch and reduce the size of the switch.
The circuit configuration generally employed to form a high-frequency switch that operates in a millimeter wave band is such that the switching element is positioned in parallel with a signal transmission path in order to reduce the transmission loss.
The most basic configuration is an SPDT (Single Pole Double Throw) switch that is formed on a GaAs substrate. The SPDT switch is separated into two branches from an input connection path via a branch point. Each branch is provided with a unit switch. The unit switch is provided with a branch signal path and a switching FET. The switching FET is shunt-connected to the branch signal path.
Each branch signal path includes a λ/4 (λ: the wavelength of an employed RF signal band) transmission path, a switching FET, and a switch-ON matching circuit. The λ/4 transmission path is connected to the branch point closely. The switching FET is shunt-connected via this transmission path to the branch signal path. The switch-ON matching circuit is positioned between a terminal end and a connection point for the switching FET. For example, the matching circuit comprises a transmission path and an open stub.
The switch-ON state is a state in which a signal propagates from an input connection path to a branch signal path output end via a unit switch. In this state, the switching FET for the unit switch is OFF. The switch-OFF state is a state in which the signal from the input connection path is shut off and does not propagate to the branch signal path output end. In this state, the switching FET for the unit switch is ON (conducting). In the following description, the terms “switch-ON” and “switch-OFF” are used to mean the above states respectively.
The SPDT switch configured as described above is such that ON/OFF control is exercised when a predetermined voltage is applied to a gate electrode of the switching FET, and that a switching operation is performed to ensure that the RF signal input from the input connection path is output from the output end of a certain unit switch.
As a well-known conventional high-frequency switch, a 5-branch switch is disclosed, for instance, at paragraphs 0013 and 0014 and FIG. 1 in Japanese Patent Laid-Open No. 2005-136630. The 5-branch switch is such that five branches are provided from an input connection path via a branch point. This switch includes a signal path and an FET. The signal path is such that each branch signal path is partly provided with a λ/4 transmission path. The FET is shunt-connected to a branch signal path that is positioned between the λ/4 transmission path and a branch signal path on the output terminal side.
As another well-known example, a single-pole double-throw (SPDT) RF switch is disclosed, for instance, at paragraphs 0011 and 0012 and FIG. 2 in JP-A-2005-515657. In this single-pole double-throw RF switch, a MOSFET transistor 23 is connected between an RF common node 25 and a first RF input node 21, and a MOSFET transistor 24 is positioned between the RF common node 25 and a second input node 22 so that the first RF input node 21 and second input node 22 are selectively coupled to the RF common node 25. Further, a shunt transistor 27 is positioned between the first RF input node 21 and MOSFET transistor 23, and a shunt transistor 28 is positioned between the second input node 22 and MOSFET transistor 24. Shunt transistor 27 and shunt transistor 28 of the single-pole double-throw RF switch work to selectively shunt the associated RF input node to a ground when the associated input nodes are not coupled to the RF common node 25 (that is, when a switching transistor (23 or 24) connected to the associated input nodes is turned OFF).
As still another well-known example, a certain configuration for an SPDT switch is disclosed, for instance, at paragraph 0012, paragraphs 0021 to 0024, FIG. 2, and FIG. 4 in Japanese Patent Laid-Open No. 1997-162602. The SPDT switch includes a common terminal and a pair of branch terminals. The branch terminals are respectively connected to the common terminal via a distributed constant path. A pin diode, FET, or other semiconductor switching element is positioned between either of the branch terminals and a grounding region. The distributed constant path has a characteristic impedance that is determined by multiplying the characteristic impedance of a high-frequency transmission system, to which the common terminal and branch terminals are connected, by the fourth root of two. Further, the distributed constant path has a transmission phase angle of 90 degrees.
In the SPDT switch having a matching circuit including an open stub, the impedance in the frequency band employed for the output node of a unit switch on the switch-ON side matches 50 ohms. However, the impedance for the output node of a unit switch on the switch-OFF side is not 50 ohms.
Therefore, when switching is made, there arises a point at which the impedance is open- or short-circuited so that resonance may occur depending on circumstances within a package containing the switch. If resonance occurs within the package, the operations of a radar or communication apparatus may become faulty. It is therefore necessary to reconfigure the paths and the like in order to avoid resonance within the package. It means that an extra design load increases. To avoid resonance, it is also necessary to increase the spacing interval between the ON and OFF terminals. It means that the cost increases due to a change in the package size.